Although considerable progress has been made in energy storage technologies, batteries remain a major obstacle to miniaturization of implantable electronics. As a result, current implantable electrical stimulation systems typically include a large impulse generator containing a titanium case enclosing the battery and circuitry used to generate the electrical pulses. The impulse generator is typically implanted within a cavity in the body such as under the clavicle, below the rib cage, in the lower abdominal region, or in the upper buttock. Electrical pulses are then delivered to a targeted nerve or muscle region via leads routed underneath the skin or through a blood vessel. Problems associated with this current approach include pocket infections, lead dislodgment, lead fracture or perforation, muscle tear due to implanting in or pulling out the leads, and limited locations for the placement of the electrodes. In addition, the lifetime of these devices is burdensomely limited, requiring periodic surgical replacement once the battery unit is depleted.
Alternatively, energy can be wirelessly transferred from an external source, but the ability to transfer power to small implanted devices and/or devices located beyond superficial depths remains challenging. Most of the known wireless powering methods for implantable electronics are based on the near-field coupling method, and these and other suggested methods suffer from a number of disadvantages. The power harvesting structure in the implanted device (e.g., the coil(s) or antenna(s)) is typically large. The largest dimension is typically on the order of a centimeter or larger. The coils external to the body in near-field coupling methods are also typically bulky and inflexible. This presents some difficulties with regard to the incorporation of the external device into daily life. The intrinsic exponential decay of the near field limits miniaturization of the implanted device beyond superficial depths (greater than 1 cm). On the other hand, the radiative nature of the far field severely limits the energy transfer efficiency. It may therefore be desirable to have devices and methods for transmitting wireless power to small implantable devices, and corresponding small implantable devices suitable for less invasive delivery methods.